High speed turbines are subject to failures which can be catastrophic to jet airplane engines and power plant electrical generators in which such turbines are used. Because turbines often are required to function for prolonged periods of time under conditions of high temperature, high pressure, and high stress, the blades of such turbines are vulnerable to progressive stress fracturing as a result of the loads imposed.
Stress fractures in turbine blades, also commonly known as flaws, in their early stages can be virtually microscopic in size. They can be invisible to visual inspection and can penetrate some distance into the blade from its surface, sometimes at a sharp angle, with only a tiny fissure at the surface. Flaws occur most commonly at the root of a blade, where flexural stress is at a maximum, but can also occur in other areas of a blade. Catastrophic failure of blades can be prevented if stress fractures are detected while still small, allowing replacement of defective blades. If permitted to grow, however, a stress fracture can lead to breaking of a blade resulting in catastrophic failure of the entire turbine.
In the case of a nuclear power plant electrical generator, such failure can be disastrous. Failure of a jet airplane engine in flight can also lead to disaster. For this reason, government and industry regulations have been established requiring the periodic inspection and testing of turbine blades.
There are numerous non-destructive examination (NDE) approaches to detecting flaws in objects. One of the oldest is x-ray imaging, in which the object to be tested is irradiated with x-rays, and variations in transmitted intensity are recorded, usually on photographic film. Voids in the object typically transmit more radiation and thus can be seen as dark areas on the film. This approach has difficulty in detecting the very small fissures typical of blade fatigue.
Another approach is the use of eddy currents to find discontinuities in metals, indicative of stress flaws. While this technique in principle is sensitive enough to find blade fatigue flaws, it requires constant adjustment of the signal and associated calibration jigs, and presents difficulty in correlation of an error signal to the location of a flaw. This technique also cannot be used on blades formed of non-conducting materials.
Another approach is to coat the surface of a ferromagnetic test object with very fine magnetic particles. Surface cracks are shown as areas in which such particles are not retained. Sub-surface cracks generally are not detected. This approach requires the test object to be ferromagnetic, which most turbine blade alloys are not, and thus magnetic NDE will not work on such blades.
Still another approach is to treat the test object with a radioactive gas. Flaws or voids in the object become filled with the gas, and subsequently a pattern of atomic particles radiating from the object can be evaluated to find such flaws or voids. Several such methods have the common goal of imaging the radiation emanating from a treated object and making that image visible.
U.S. Pat. No. 3,499,319, issued Mar. 10, 1970 to Figueroa, describes a method for treating an object with radioactive krypton gas (Kr-85) and then exposing the object to photographic film for up to three days to detect radiation emanating from voids in the object.
U.S. Pat. No. 3,621,252, issued Nov. 16, 1971 to Eddy, involves the use of a fiber optics bundle in detection of flaws by radioautography. The ends of the fibers are coated with a phosphor which emits light when exposed to sufficient radiation. These ends are exposed to a video camera, and the video signals are electrically amplified and used to provide a television picture of the image formed at the end of the fiber bundle.
U.S. Pat. No. 3,891,844, issued Jun. 24, 1975 to Gibbons, discloses the use of a photographic emulsion liquid coated and then chill-set on the surface of a test object previously treated with Kr-85. The emulsion is supposed to seal the residual krypton gas in the flaws and become photographically exposed in areas of high radioactivity. The exposure pattern is made visible by photographic development of the coated object.
Krypton-85 is the gas of choice in the foregoing radiation-detecting methods. Its advantages are that it is an unreactive gas having a relatively small molecular diameter of approximately 3 .mu.m, and therefore able to penetrate extremely narrow fissures. Its drawbacks are a relatively long half-life of ten years and its low-energy beta emission. The long half-life results in a low specific activity and therefore a relatively low signal strength. Its beta emission is capable of penetrating most metal objects only a small distance and therefore radiation from relatively deep fissures is highly attenuated and not reliably detected by the above methods.